The present invention relates generally to optical transmission systems, and more particularly to an optical transmission equipment and system that transmit a plurality of optical signals having different wavelengths over a single optical fiber and a method for optically amplifying wavelength division optical multiplexed signals.
As a means of making optical transmission systems have more transmission capacity, Wavelength Division Multiplexing (WDM) transmission systems that transmit a plurality of optical signals having different wavelengths over a single optical fiber are in practical use. Optical fiber amplifiers (hereinafter referred to as optical amplifiers) such as an Erbium-Doped Fiber Amplifier (EDFA) have such characteristics as to enable simultaneous amplification of optical signals in a wide spectrum of wavelengths. By combining WDM with optical amplifiers, a plurality of optical signals with different wavelengths are amplified at a time and thus economical, large-capacity, and long-range transmission can be realized in simple structure.
In such WDM and optical amplification transmission manner, however, the power level of light beams input to a fiber is so high that it has posed a problem of nonlinear effect that deteriorates transmission characteristics. One typical nonlinear effect is Stimulated Raman Scattering (SRS) that causes problems of over-loss and over-gain.
The SRS is a nonlinear optical process in which part of input signals to an optical fiber act as excitation light and interact with lower-frequency signal light beams in the fiber, causing energy to move from higher-frequency signal light beams to lower-frequency signal light beams. The SRS takes place in all optical fibers and its influence degree depends on the optical fiber type and the frequency difference between optical signals for which such energy moving occurs.
The moving energy increases in proportion to the sum of the intensity of all light beams output by WDM. The more the number of wavelengths accommodated by WDM equipment and the wider the wavelength bands accommodated, the moving energy will be higher and the manifestation of its influence will be significant. Moreover, the longer the distance of transmission, the manifestation of its influence will be more significant.
For the WDM equipment that accommodates signals with different wavelengths, such a problem arises that the light output intensity varies by wavelength at the receiving end and consequently varying optical signal-to-noise ratios (OSNR) are measured for different wavelengths. Because light output intensity variation by wavelength also occurs during transmission over a fiber, a further problem arises that self phase modulation, chromatic dispersion and frequency chirp in combination cause different receiving waveform distortions for different waveforms, transmission errors, and transmission distance variance by wavelength.
In conventional WDM equipment operating condition, since the number of wavelengths of optical signals accommodated and wavelength bands were relatively small, the influence of SRS was not so much. However, due to recent communications traffic expansion, request has been made to the WDM equipment for increasing the number of wavelengths to be accommodated and bandwidth extension. Thus, it now becomes impossible to be regardless of the influence of SRS. Some recent academic report has pointed out that, if the WDM equipment accommodates all wavelengths assumed by its initial design, the influence of SRS becomes so great as to make transmission impossible.
Some previous transmission systems using the WDM equipment have been proposed with consideration given to the influence due to the dependency on the gain tilt of an optical amplifier integrated into the system and the wavelength loss of a transmission path; for example, a method for obtaining the minimum OSNR by suppressing the gain tilt occurring in optical amplifiers, set forth in JP-A No. 223136/1996, and a method for minimizing the optical signal power level variance and the OSNR at the receiving end, set forth in JP-A No. 55812/1999.
The above-mentioned previous inventions are effective if optical signals input to optical amplifiers always have constant light output intensity, that is, transmission loss and optical amplifier gain tilt do not vary as the number of wavelengths increases or decreases. However, there is no consideration to a case that the light output intensity varies, according to the number of wavelengths accommodated and the wavelength bandwidth.
In JP-A No. 183818/2000, a method for making the OSNR at the receiving end constant by adjusting the intensity (pre-emphasis) of the optical signals having different wavelengths at the sending end has been proposed. However, if the number of wavelengths accommodated is great in a wide spectrum, extremely great signal output level variance may occur so as to exceed the level variance that can be corrected by the pre-emphasis.
There is a need to provide a wavelength division multiplexing optical transmission system that is capable of rectifying the signal-to-noise ratio variance of optical signals received at the receiving end. Although there may be some possible factors causing such variance, particularly, the present invention is intended to rectify the variance caused by Stimulated Raman Scattering (SRS) that optical signals being transmitted over a fiber encounter.
It is desirable to provide a method of estimating beforehand the signal-to-noise ratio variance of optical signals to be received at the receiving end, based on the intensity characteristics of optical signals output from optical amplifiers, in order to rectify such variance caused by SRS.
Yet, another object of the present invention is to provide a method of configuring the above system with optical transmission equipment having optical amplifier output characteristics to rectify the signal-to-noise ratio variance of optical signals to be received at the receiving end, based on the estimated variance.